This invention relates to a method for producing vinyl norbornene preventing the formation of by-products in Diels-Alder reaction. More particularly, the invention relates to a method for producing vinyl norbornene at a high yield preventing the formation of by-products in Diels-Alder reaction of cyclopentadiene and butadiene which is characterized in that the reaction is carried out in the presence of p-phenylenediamine compounds.
In Diels-Alder reaction, the reaction of cyclopentadiene and butadiene is brought about to produce vinyl norbornene. Since this reaction proceeds thermally, it is desirable to raise the reaction temperature to some degree, for example, 70.degree. to 250.degree. C. In higher temperatures, however, though some concentration of butadiene and cyclopentadiene exists, the yield of vinyl norbornene is reduced with reaction time.
One of the causes of this phenomenon is that in the literature of A. P. Plate [Zhur. Obshckei Khim., 31 131 (1961)] vinyl norbornene (I) rearranges thermally to tetrahydroindene. But the inventor has found the other one wherein vinyl norbornene is so reactive that it reacts with cyclopentadiene to produce Diels-Alder reaction by-products having 2 to 4 cyclopentadiene units and one butadiene unit as follows: ##STR1## That is to say, the yield of vinyl norbornene (I) which is the object of the present invention is remarkably reduced by undesirable reactions in which cyclopentadiene is further added to vinyl norbornene (I) by a Diels-Alder reaction.
As the method for preventing the undesirable reaction at a higher temperature, those of British Pat. No. 923,462 and U.S. Pat. No. 3,201,484 are heretofore known. In the former method, organocobalt compounds are added, and in the latter method, organonickel compounds are added. While both the organocobalt and organonickel compounds are unstable in air, in addition they are weak in thermal influence, so that the use of these compounds presents many difficulties when the reaction is practically carried out.
On the other hand, it is known in U.S. Pat. No. 2,686,208 that when a cyclic compound is synthesized by using butadiene, p-phenylenediamine is added to prevent the formation of open chain polymers of butadiene. It is also known in U.S. Pat. Nos. 2,373,715 and 2,413,256 that in the case of refining and separating of butadiene, p-phenylenediamine is used as an inhibitor in order to prevent the formation of high polymers of butadiene. Further it is also known in U.S. Pat. No. 2,972,640 that p-phenylenediamine is added to preclude the formation of open chain diolefins which are a polymer of butadiene.
All of these prior patents disclose the prevention of the formation of butadiene polymers, but do nothing about the prevention of the formation of any polycyclic compound which is undesirably formed by the addition of 2-4 rings of cyclopentadiene to vinyl norbornene that is intended in the present invention.